X-ray image intensifier system



Dec. 25, 1956 s. HANSEN 2,775,719

X-RAY IMAGE INTENSIFIER SYSTEM Filed June 30, 1955 2 Shee'cs-Sheze'L l Dec. 25, 1956 s. HANSEN 2,775,719

x-RAY IMAGE INTENSIFIER SYSTEM Filed June 30, 1953 2 Sheets-Sheet 2 4f Irfan/ix United States Patent O X-RAY IMAGE nsTENsIFmR SYSTEM Siegfried Hansen, Los Angeles, Calif., assignor, by mesne assignments, to Hughes Aircraft Company, a corporation of Delaware Application June 30, 1953, Serial No. 365,161

5 Claims. (Cl. 315-11) This invention relates to X-ray image intensifier systems and more particularly to an X-ray image intensifier system incorporating storage tube and negative feedback principles to produce an X-ray image of a subject under examination who or which need only be subjected to X-rays for short periods of time at periodic intervals.

As applied to animate objects, it is commonly known that X-rays have the capacity to damage or destroy living tissue. In X-ray therapy, use is made of this fact by exposing morbid tissue to these rays for such periods and at such intensities that in total, it will receive a certain, heavy dose of the X-rays. ln X-ray diagnostics, on the other hand, care must be taken that patients receive the smallest possible dose of X-rays during examination so that they will not suffer any injurious consequences, even as a result of repeated examinations.

For a given duration of an examination, which may be at most only a few minutes, limitation of the total dosage of X-rays means limitation of the dosage rate to which the patient may be subjected with the concomitant limitation, dependent upon the absorption of the rays by the patient, of the brightness that can be obtained from the X-ray screen. Using the conventional method of iluoroscopy, therefore, the radiologist must do the best he can with an X-ray image of very low brightness. For example, in order for a radiologist to make a good visual assessment at such low brightness values, he must first remain in darkness for at least minutes so that his eyes may be adapted to the low brightness level. Even so, his visual acuity and contrast sensitivity still remain fairly low. Hence, thel low intensity of brightness is a very serious limitation for the diagnosis of conditions involving slight abnormalities whch often reveal only a low contrast. It is therefore quite obvious that higher image brightnesses have been an ever-present need since the inception of X-ray diagnostics.

The X-ray image intensifier system of the present invention, reduces the exposure of the patient to X-rays and at the same time provides improved image brightness. This is accomplished by utilizing a television type camera tube adapted to produce an electrical signal representative of an X-ray image. This tube includes a target which comprises a thin sheet of glass having a uoroscopic screen disposed on one side thereof and a transparent conductive layer and a photo-conductive layer disposed on the other side. A collector grid is positioned adjacent to and in register with the outer surface of the photoconductive layer. Means are provided for periodically scanning this surface of the photoconductive layer with a high energy electron beam of elemental cross sectional area.

In its operation, the X-rays, after passing through the object to be examined, are received on the fluoroscopic screen Where they are converted to light. This light initiates a photoconductive process which produces a charge pattern representative of the X-ray image on the scanned surface of the photoconductive layer. A

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unity high gain` negative feedback circuit is connected between the tarnsparent conductive coating and the collectorgrid so that an electrical signal is produced that is an exact replica of the charge pattern scanned by the electron beam. This feedback circuit is disclosed in a copending application entitled, Feedback Circuits for Storage Tubes, by Siegfried Hansen, Serial No. 198,610, iiled December 1, 1950. With the disclosed X-ray system of the present invention, the object may be exposed to the X-rays for 0.1 second periods at 10 second intervals, thus greatly reducing the dosage of the X-rays. This dosage will depend, of course, on the rate of decay of the charge pattern on the screen of the storage tube and the density or opacity of the object to the X-rays. In the case Where there is motion or change in successive X-ray images, means is provided to erase the charge pattern just prior to each subsequent exposure of the object being examined to the X-rays.

The X-ray image of the object is viewed on a conventional cathode ray tube by modulating the intensity of its electron beam in accordance With the replica of the charge pattern while scanning the electron beam over the viewing screen of the tube in synchronism with the electron beam of the camera tube.

It is thus seen that the X-ray image intensifier system has the advantage of considerably reducing the dosage of X-rays to a patient While at the same time producing an image having an improved brightness level that may be viewed by the radiologist without the necessity of adapting his eyes to a very low level of brightness of the image. The disclosed system also enables the radiologist to view the X-ray image at a position remote from the source of X-rays so as not to endanger his health.

An alternate embodiment of the present invention incorporates a camera tube disclosed by the applicant in a copending application entitled, An Electronic Camera Tube, Serial No. 365,162, led, June 30, 1953. This tube includes a flood gun which directs high energy electrons uniformly over the surface of its photoconductive layer so as to charge it in a negative direction. This charging in a negative direction by the high energy flood electrons is used to off-set charging in a positive direction by a photoconductive process of increased sensitivity and thus enable the net charging eiected by the high energy electron beam of elemental cross section to be in a positive direction, thereby minimizing the redistribution of secondary electrons.

It is therefore an object of this invention to provide an X-ray image intensier system capable of producing an X-ray image of improved brightness.

Another object of this invention is to provide an X-ray image intensifier and image-storing system capable of producing an X-ray image having improved brightness in response to X-rays projected through a patient for short periods of time at periodic intervals.

Still another object of this invention is to provide a continuous bright X-ray image in response to X-rays projected through an object to be examined for short periods of time at periodic intervals. j

A further object of this invention is to provide an X-ray image intensifier system incorporating an electronic camera tube to produce an electrical charge pattern representative of an X-ray image, and utilizing a negative feedback circuit in cooperation with secondary electron emission means to produce an electrical signal representative of, but not affecting, the electrical charge pattern.

Figs. 1 and 2 are diagrammatic sectional views of iirst and second embodiments of the X-ray image intensifier system, respectively. l

Referring to Fig. 1 a rst embodiment of the VX-ray image intensifier system of the present invention comprises an X-ray camera tube 10, a scanning signal generator 12, a conventional cathode ray picture tube 14 and a feedback circuit 16. y

X-ray camera tube of the Vsystem comprises an evacuated envelope 20, which in its left portion, as viewed in the ligure, has an electron gun 24, including a cathode 25, for producing an electron beam of elemental cross sectional area and deflecting plates 26 disposed about the path of the electron beam as it emerges from electron gun 24. Cathode of electron gun 24 is maintained at a negative potential of the order of -1000 Volts with respect to ground by a connection to the negative terminal of a suitable potential source 23, the positive terminal of which is Connected to ground. A collector grid 27 and a target electrode 23 are disposed in the enlarged right portion of evacuated envelope 20, as shown in the figure. An electrode 29 provides a suitable drift space between deecting plates 26 and collector grid 27 and may comprise any suitable conductive material disposed on the inner surface of evacuated envelope 20 between deflecting plates 26 on the one extremity and collector grid 27 on the other. Electrode 29 is maintained at ground potential by means of a suitable connection thereto.

As previously mentioned, target electrode 28 and collector grid 27 are disposed in the right portion of evacuated envelope 20 as viewed in the ligure. Target electrode 28 comprises a thin glass plate 31 having a transparent conductive coating 32 such as, for example, tin oxide, disposed on the side exposed to electron gun 24. A conventional method of providing a transparent conductive coating of tin oxide onl glass is to expose the surface of the glass to stannous chloride vapors in the presence of oxygen.

A photoconductive layer 33 is then applied to the conductive coating 32. ln this particular tube, this material is of the type that transfers charge by actual ow of electrons as distinguished from the migration of holes, that is, regions having a deficiency of electrons. A suitable material of this type is antimony trisultde. The thickness of this photoconductive layer 33 is of the order of from 5 to l0 microns. Target electrode 28 is disposed so that the outer surface of photoconductive layer 33 is exposed to the electron beam produced by electron gun 24, the inner surface of photoconductive layer 33 being in contact with transparent conductive coating 32. This transparent conductive coating 32 is maintained at a quiescent potential of v the order of -100 volts with respect to ground by a connection through a resistor to the negative terminal of a potential source 36, the positive terminal of which is connected to ground.

A uoroscopic X-ray screen 30 is disposed on the surface of thin glass plate 31 that is opposite from the surface on which the transparent conductive coating 32 is disposed. The fluoroscopic X-ray screen 30 may include, for example, zinc sulphide activated with silver to obtain a blue fluorescence when excited by X-rays. The glass plate 31 is thin in order to minimize diffusion of the light between the X-ray screen 30 and the photoconductive layer 33, a representative thickness being of the order of 1 millimeter or less.

The collector grid 27, as shown in Fig. l, comprises a conductive mesh having of the order of from 200 to 250 wires per inch. It is to be noted that collector grid 27 may assume other forms such as, for example, an annular type electrode, and still perform the same function. Collector grid 27 is disposed adjacent to and in register with the surface of photoconductive layer 33 that is exposed to the electron beam. The quiescent potential of collector grid 27 is maintained at ground potential by a connection through a resistor 37 to ground. Current variations through resistor 37 provide an output signal which is available at a terminal 38.

The output signaly from the camera tube 10 appearing at terminal 38 is impressed on the input circuit of a video amplitier 41, the output circuit of which is connected to terminals 42 and 43, terminal 43 being connected to ground. The output voltage of video amplifier 41 appearing at terminals 42, 43 is fed back to transparent conductive coating 32 of target electrode 28. This is accomplished by means of a connection from terminal 42 through a contact 105 of a relay`107 to transparent conductive coating 32. The output voltage of video amplitier 41 is also impressed on the input circuit of an additional video amplifier 45 by means of connections thereto from terminal 42 through contact 195 of relay 107 and from terminal 43. The gain of video amplitier 41 may be of the order of 5000, while the gain of video amplifier 45 is adjustable, the actual gain required being dependent on the characteristics of the cathode ray picture tube 14. Relay 107 is energized by means of a connection to a synchronizing pulse source 108 which has a lead 109 that goes to the source of X-rays for the purpose of` applying energizing signals thereto.

Cathode ray tube 14 comprises an evacuated envelope 51, which in its right portion, as viewed in the figure, has an electron gun 52 for producing an electron beam of elemental crosssectional area and deecting plates 55 disposed about the path of the electron beam. Electron gun 52 includes a cathode 53 and an intensity grid 54 for controlling the intensity of the electron beam. A conventional cathode ray tube screen 56 maintained at ground potential is disposed on the inner surface of the enlarged left portion or evacuated envelope 51. Cathode 53 is maintained at a potential of the order of from 3000 to 10,000 volts negative with respect to the potential of screen 56 by a connection to the negative terminal of a potential source 57 which has its positive terminal connected to ground. Control grid 54 is biased with respect to cathode 53 by a connection through a resistor 58 to the negative terminal of an adjustable potential source 59, the positive terminal of which is connected to cathode 53. The output circuit of video amplifier 4S is connected through a capacitor 61 to intensity grid 54so that the output signals from video amplier 45 control the intensity of the electron beam produced by the electron gun 52.

The output circuit of scanning signal generator 12 is connected so as to impress scanning signals on deflecting plates 26 of the X-ray camera tube 10 and deliecting plates 55 of the cathode ray picture tube 14. Thus, the electron beams produced by electron guns 24 and 52 both scan corresponding rasters on their respective f targets.

In operating the X-ray image intensifier system of the present invention, an X-ray beam is periodically projected through a body or object 62 to be examined for short intervals of time on to the fluoroscopic X-ray screen 30 of the X-ray camera tube 10. The durationk Inasmuch as the quiescent potential of collector grid 27 is ground, the affect of scanning the surface of photoconductive layer 33 with the electron beam of elemental cross sectional area is to charge the entire scanned surface to an equilibrium potential that is several volts positive with respect to ground potential in aCcOrdance with secondary electron emission phenomenon. Thus, a positive potential gradient in a direction towards the scanned surface is established across photoconductiveV layer 33. That is, the surface of photoconductive layer 33 in contact with transparent conductive coatingp32 is maintained at Volts with respect to ground While the surface of layer 33 scanned by the electronv beam is initially charged to substantially ground potential. Since photoconductive layer 33 is composed of the type'of material that conducts electric charge by actual ow of electrons, the electrons liberated by the light produced by X-rays incident on the iiuoroscopic X-ray screen 30 flow through the photoconductive layer 33 to the more positive surface scanned by the electron beam. The electrons arriving on the scanned surface of layer 33 thus charge elemental areas of the surface in a negative direction by an amount that is proportional to the light intensity initiating the photoconductive process. An electrical charge pattern is thus produced that is representative of the X-ray image projected on iluoroscopic X-ray screen 30.

The normal manner in which a signal is produced from a charge pattern of this type is to scan the surface of photoconductive layer 33 with an electron beam, thereby charging the surface in a positive direction back to the equilibrium potential. Since the charging of the surface is in a positive direction, all the electrons in excess of the electrons in the electron beam which are collected by the grid 27, are indicative of the charge pattern. The electrons collected by grid 27 flow through resistor 37 to ground to produce an electrical signal representative of the charge pattern which is available at terminal 38.

In the disclosed X-ray image intensifier system, the aforementioned charging of the surface back to the equilibrium potential does not take place because of the negative feedback arrangement. More particularly, the electrical signal representative of the charge pattern -that is available at terminal 38 is impressed on the input circuit of high gain video amplifier 41. The output signal from video amplifier 41 is .applied through terminal 42 through contact 105 of relay 107 to transparent conductive coating 32 in such a manner as to tend to reduce the input signal to the amplier 41 to zero. This is accomplished by virtue of the fact that elemental areas of the scanned surface of photoconductive layer 33 may be regarded as each having capacitance to the transparent conductive coating 32, thus enabling the potential of a scanned elemental area of photoconductive layer 33 to be controlled by the potential applied to transparent conductive coating 32. Hence, video amplifier 41 will produce a signal at terminal 42 that tends toshift the potential of the particular elemental area of the surface of photoconductive layer 33 being scanned by the electron beam to the equilibrium potential. Thus, instead of charging the elemental areas of the scanned surface from the potentials constituting the charge pattern to the equilibrium potential, the equilibrium potential is shifted to conform with the potentials of the charge pattern. An electrical signal having substantially the same variations in amplitude as the scanned portions of the charge pattern is produced at terminal 42 While the charge pattern is substantially unaffected.

However, since the quiescent potential of the scanned surface of photoconductive layer 33 and of the transparent conductive coating 32 remans xed and the direct current gain of video amplifier 41 is zero, the overall scanned surface must be charged by an amount equal to the total photoconductive current. As previously specified, the photoconductive current charges the scanned surface of layer 33 in a negative direction, hence, any shift in the potential level of the charge patternwill be in :a positive direction. Thus, the photoconductive process is oriented so that charging of elemental areas of the scanned surface by the electron beam is 'in a positive direction. Charging an elemental area in a positive direction tends to minimize redistribution of the secondary electrons released from an elemental area scanned by the electron beam. That is, when charging an elemental area in a positive direction by means of the electron beam, secondary electrons not `attracted to the collector grid 27 will return to the more positive area being scanned by the electron beam rather than to adjacent ar'eas'lof the surface, presuming that the initial potential of the scanned elemental area and adjacent areas were substantially equal. Secondary electrons that are redistributed to the adjacent areas would have the same effect, of course, .as electrons arriving there as a result ofthe photoconductive process. The phenomenon comprising the returning of secondary electrons toareas adjacent to the area scanned by the electron beam is generally referred to as the redistribution of electrons effect. This effect causes loss of contrast in the electrical signal representative of the X-ray image and may be minimized by using secondary electron emission means to charge the surface of photoconductive layer 33 in a positive direction only.

An electrical signal is thus produced which appears at terminal 42 that is a replica of the charge pattern produced by the photoconductive process on the photoconductive surface. This electrical signal is impressed on the input circuit of video amplifier 45 where it is ampliiied to a suitable amplitude and impressed through capacitor 61 on intensity grid 54 of the picture tube 14 to control the intensity of the electron beam produced by electron gun 52. Since similar scanning signals are impressed on the deiecting plates 26, 55 of both tubes, the beam from electron gun 52 produces an X-ray image of body 62 on the viewing screen 56 of the cathode ray picture tube 14. vPicture tube 14, of course, can be located in a location remote from the source of X-rays so as not to continually expose the radiologist to stray X-rays that possibly are present about the equipment.

Inasmuch as there may be changes in successive X- ray images, it may be desirable to erase the existing charge pattern before a new one is produced. This is accomplished by having synchronizing pulse source 108 momentarily energize relay 107 while the electron beam scans the surface of photoconductive layer 33 at least once just prior to energizing the source of X-rays. Energization of relay 107 opens contact 105 to remove the output of video amplifier 41 from transparent conductive coating 32 and from video amplifier 45. The opening of contact 105 removes the feedback signal from coating 32 to effect erasing .of the charge pattern by virtue of the fact that coating 32 will now remain at a fixed potential while the surface of photoconductive layer 33 is-scanned by the electron beam. Inasmuch as a considerable output signal will be produced at the input of amplier 41 during this erasing process, the output of amplifier 41 is also disconnected from amplifier 45 by the opening contact 105 so as not to impress a largesignal on grid 54 of cathode ray tube 14.

An alternate embodiment, illustrated in Fig. 2, of the X-ray image intensifier `system of the present invention incorporates an X-ray camera tube wherein a photoconductive process of increased sensitivity is used that effects the conduction of electric charges through the photoconductive layer in the form ofl holes towards a surface that is scanned by an electron beam of elemental cross sectional area and uniformly bombarded by high energy ood electrons. This X-ray camera tube is vdisclosed, as previously mentioned, in a copending application for patent entitled, An Electronic Camera Tube. In this embodiment, the ilood gun providing the high energy flood electrons in the camera tube is energized simultaneously with the energization of the source of X-rays, other phases of operation of the system being substantially the same as for the system shown in Fig. l.

Referring to Fig. 2, the alternate embodiment ofthe X-ray image intensifier system comprises, as before, a scanning signal generator 12, a conventional cathode ray`picture tube 14, a feedback circuit 16, an electronic camera vtube 70 inlieu of the X-ray camera tubeV 10 and, in addition,Y a pulse source 71 to synchronize the erasing process' with the energization of a source of` X- rays 72 and theood electrons in camera tube 70.'

ACamera tube 70 of the system comprises an evacuated envelope 80, which in its left portion, as viewed in the gure, has a flood gun S1 including a cathode 82 and an intensity grid 83, an Aelectron gun 84 including a cathode 85, for producing an electron beam of elemental cross sectional area, and deflecting plates `86 disposed about therpath of the electron beam as it emerges from the gun 84. Cathode 85 of electron gunY 84 is maintained at a negative potential of the order of 1000 volts with respect to ground by a connection to an appropriate terminal of a potential source 87, the positive terminal of which is connected to ground. A collector grid 88 and a target electrode 89 are disposed in the enlarged right portion of evacuated envelope 80, as shown in the figure.

The cathode 82 of flood gun 81 is maintained at a i,

potential that is dependent on the secondary electron emission characteristics of the surface of target electrode 89 that is exposed to the action of the ood electrons. More particularly, cathode 82 is maintained at a potential that is sufliciently negative with respect to the potential of this exposed surface to cause the electrons to impinge on the surface at such a high velocity that fewer secondary electrons are released from the surface than incident on the surface. This potential may be of the order 5000 volts with respect to ground and is impressed on cathode 82 by means of a connection to the negative terminal of potential source 87. A potential is normally maintained on intensity grid 83 of flood gun 81 that is suiciently negative with respect to the cathode 82 so as to stop the flow of ood electrons. This is accomplished by connecting intensity grid 83 through a resistor 91 to the negative terminal of a variable source of potential 92 which has its positive terminal connected to cathode 82. Source of potential 92 provides a voltage that may be varied, for example, from 200 to 500 volts.

An electrode 93 provides a suitable drift space between ood gun 81 and deliecting plates 86 on the one extremity and collector grid 88 on the other and may be composed of any suitable conductive material disposed on the inner surface of evacuated envelope 80 as shown in the figure. Electrode 93 is maintained at ground potential by means of a suitable 'connection thereto.

As previously mentioned, target electrode 88 and collector grid 89 are disposed in the enlarged right portion of evacuated envelope 80 as viewed in the figure. Target electrode S9 is the same as target electrode 23 in the X-ray camera tube 10 of Fig. 1, except that a photoconductive layer 94 composed of the red vitreous form f' of selenium is employed in lieu of photoconductive layer 33 which, by way of example, was composed of antimony tn'sulde. The material of which photoconductive layer 94 is composed, transfers electric charge in the form of holes, that is, in the form of regions having a deficiency of electrons that migrate through the material in the direction of a negative potential gradient. Thus, target electrode 89 comprises a thin sheet of glass 31a on which having a lluoroscopic screen 30a on one surface and a transparent conductive coating 32a and the photoconductive layer 94 on the other.

Transparent conductive coating 32a is maintained at a quiescent potential of the order of +50 volts with respect to ground by means of a connection through a resistor 95 to the positive terminal of a potential source 96, which has its negative terminal connected to ground.

Collector grid 88, being similar in construction to the collector grid 27 shown in Fig. 1, comprises a conductive mesh having of the order of from 200 to 250 per inch. Collector grid 88 is disposed adjacent to and in register with the surface of photoconductive layer 94 that is exposed to the electron beam and is maintained at quiescent ground potential by a connection thereto through resistor 37.

As in the case of the X-ray image intensifier system illustrated in Fig. l, current variations through resistor 37 provide an output signal which is available at terminal 38. This output'signal from the camera tube 70 is impressed on the input circuit of video ampliler 41 which has its output circuit connected to terminals 42, 43. The output voltage appearing at these terminals 42, 43 is fed back to transparent conductive coating 32a by means of a connection thereto through contact 105 of relay 107 to terminal 42. Scanning signal generator 12 energizes deecting plates 86 of camera tube 70and deilecting plates 55 of cathode ray picture tube 14 with similar scanning signals. The remaining connections between the negative feedback circuit 16 and cathode ray picture tube 14, are the same as for the system of `Fig. l.

The pulse source 71 generates signals for periodically energizing the source of X-rays 72 simultaneously with the flood gun 81 of camera tube 70 in synchronism with signals for energizing relay 107. The signals from pulse source 71 are applied to X-ray source 72 over a lead 93. In the case of intensity grid 83, however, it is necessary that the density of the ileod electrons be adjusted to a value dependent on the average rate of charging effected by the photoconductive process. Accordingly, the signals from pulse source 71 are applied through a potentiometer 99 and a capacitor 101 to intensity grid 83. An adjustable contact of potentiometer 99 determines the magnitude of the signal applied to intensity grid 83 and hence may be used to adjust its bias with respect to cathode 82 during the periods that tlood gun 81 is energized and thereby determine the density of the flood electrons.

In the operation of the X-ray image intensifier system, pulse source 71 energizes an X-ray beam for short periods of time at periodic intervals. This X-ray beam is projected through the body 62, to be examined, on to the uoroscopic X-ray screen 30a of target electrode 39. The X-rays incident on fluoroscopic screen 30a are converted to light which liberates electrons from the surface of photoconductive layer 94 in contact with transparent conductive coating 32a to initiate the photoconductive process.

As in the case of the system of Fig. 1, the affect of scanning the surface of photoconductive layer 94 with the electron beam produced by electron gun 84, while collector grid 94 is maintained at ground potential is to charge the scanned surface to an equilibrium potential that is several volts positive with respect to ground. Thus, a negative potential gradient is established across photoconductive layer 94 in going from the surface in contact with transparent conductive coating 32a to the scanned surface, as the coating 32a is maintained at +50 volts with respect to ground potential. Because of this negative potential gradient, the electrons liberated by the light produced by the X-rays incident on the fluoroscopic screen 30a are conducted away leaving regions in the photoconductive layer 94 which have a deficiency of electrons, these regions being normally referred to as holes.

Since these holes are essentially positive charges, they are attracted towards the scanned surface of photoconductive layer 94 in the direction of the negative potential gradient to produce a charge pattern representative of the X-ray image. However, since the holes constitute positive charges, this charge pattern will necessarily be positive with respect to the aforementioned equilibrium potential. Maintaining the average potential of the charge pattern at ground potential, however, entails shifting it in a negative direction which, as previously described, tends to cause a redistribution of the secondary electrons released from the areas scanned by the electron beam.

To eliminate this ditlculty, the charge pattern is shifted uniformly in a negative direction by the action of the ood electrons. The velocity at which the ood electrons impinge on the surface of photoconductive layer 94 is suiiciently high so that fewer secondary electrons are released from the surface than incident thereon thus charging it in a negative direction. The density of the flood electrons is adjusted during the charging period by means of the adjustable contact 100 of potentiometer 99 so that the charging in a negative direction offsets the average charging in a positive direction by the photoconductive process.

The operation of the feedback arrangement and the remaining portion of the system is the same as for the X-ray image intensifier system of Fig. l.

What is claimed as new is:

1. The method of producing a continuous visual presentation of a periodic X-ray image with the aid of a photoconductive layer having first and second surfaces with capacitance therebetween, said first surface being in contact with a transparent conductive coating and said second surface possessing secondary electron emission characteristics, said method including the steps of projecting the periodic X-ray image towards said first surface of said photoconductive layer, converting the X-ray image into a light image in a plane contiguous to said rst surface to release electrical charge from each elemental area of said first surface in proportion to the light intensity thereon, directing the electrical charges to corresponding elemental areas of said second surface to produce a charge replica of the X-ray image, producing an electron beam of elemental cross sectional area, scanning said second surface with said electron beam to liberate more secondary electrons from each scanned elemental area than beam electrons incident thereon, collecting the secondary electrons from said second surface at a rst quiescent potential to produce a first electrical signal representative of said charge replica and to discharge said second surface to substantially said first potential, amplifying alternating components of said rst electrical signal to produce a second electrical signal, impressing said second electrical signal on said transparent conductive coating to shift the potential of the elemental area of said second surface being scanned by said electron beam substantially to said rst potential by means of said capacitance whereby said charge replica is not affected, said second electrical signal being a reproduction of the scanned areas of said charge replica capable of being converted into a visual presentation of the X-ray image.

2. The method of producing a continuous visual presentation of a periodic X-ray image with the aid of a photoconductive layer with secondary electron emission characteristics having first and second surfaces with capacitance therebetween, said iirst surface being in contact with a transparent conductive coating, said method including the steps of generating an electron beam of elemental cross sectional area, scanning said second surface with said electron beam to liberate more secondary electrons from said second surface than beam electrons incident thereon, collecting the secondary electrons at a predetermined quiescent potential to charge said second surface substantially to said quiescent potential, periodically directing high energy electrons uniformly over the area of said second surface to charge each elemental area thereof in a negative direction at a predetermined rate, projecting an X-ray image towards said rst surface of said photoconductive layer simultaneously with the directing of said high energy electrons over the area of said second surface, converting the X-ray image into a light image in a plane contiguous to said first surface to release photo electrons from each elemental area thereof in proportion to the light intensity thereon, removing the released photo electrons away from said rst surface to produce holes, directing the holes towards said second surface to charge each elemental area thereof in a positive direction at a rate less than said predetermined rate and proportional to the light intensity on the corresponding elemental area of 1G Y said first surface thereby producing a charge replica of the X-ray image whereby the secondary electrons collected at said quiescent potential constitute a rst electrical signal representative of said charge replica, amplifying alternating components of said first electrical signal to produce a second electrical signal, and impressing said second electrical signal on said transparent conductive coating to shift the potential of the elemental area of said second surface being scanned by said electron beam substantially to said quiescent potential by means of said capacitance whereby said charge replica is not affected and said second electrical signal is a reproduction of the scanned areas of said charge replica.

3. An X-ray image intensifier system for presenting a continuous visual presentation of a periodic X-ray image, said system comprising means for generating an electron beam of elemental cross sectional area; a target element including a thin glass pane, a fluoroscopic screen disposed on one side thereof for converting the X-ray image into a light image, a transparent conductive coating disposed on the other side thereof, and a photoconductive layer with secondary electron emission characteristics having first and second surfaces with capacitance therebetween, said first surface being in contact with said conductive coating and said second surface being exposed to said electron beam; means for scanning said second surface with said electron beam to liberate more secondary electrons than beam electrons incident thereon; means for collecting said secondary electrons at a first quiescent potential level to charge said second surface substantially to said first potential level; means for maintaining said conductive coating at a second quiescent potential level different from said first potential level t-o produce a potential gradient .across said photoconductive layer whereby the light intensity of elemental `areas 'of the light image releases electric charge from corresponding elemental areas of said first surface in proportion to the light intensity thereon and said potential gradient directs said electric charges to said second surface to produce a charge replica of said light image, the secondary electrons collected at said first potential level constituting a first electrical signal representative of the light image; means for amplifying said rst electrical signal to produce a second electrical signal; means for impressing said second electrical signal on said conductive coating to shift the poten-tial of `the elemental area of said second surface being scanned by said electron beam substantially to said first quiescent potential whereby said char-ge replica is not affected and said second electrical signal is a reproduction of the scanned areas of said charge replica capable of being converted into a visual presentation of said X-ray image.

4. An X-ray image intensifier system for presenting a continuous visual presentation of a periodic X-ray image, said system comprising means for generating an electron beam of elemental cross sectional area; a target element including a thin glass pane, a uoroscopic screen disposed on one side thereof for converting the X-ray image into a light image, a transparent conductive coating disposed on the other side thereof, and a photoconductive layer with secondary electron emission characteristics having first and second surfaces with capacitance therebetween, said first surface being in contact with said conductive coating and said second surface being exposed to said electron beam; means for scanning said second surface with said electron beam to liberate more secondary electrons than beam electrons incident thereon; means for collecting said secondary electrons at a first quiescent potential level to charge said second surface substantially to said first potential level; means for main-'taining said conductive coating at a second quiescent potential level negative with respect to said first potential level to produce a potential gradient across said photoconductive layer whereby the light intensity of elemental areas of the light image releases photo electrons from corresponding elemental areas `of said rst surface in proportion to the light intensitythereon and said potential gradient directs said released photo electrons to said second surface to produce a charge replica of said light image, the secondary electrons collected at said first potential level constituting a first electrical signal representative of the light image; means for amplifying said first electrical signal to produce a second electrical signal; means for impressing said second electrical signal on said conductive coating to shift the potential of the elemental area of said second surface being scanned by said electron beam substantially to said first quiescent potential whereby said charge replica is not affected and said second electrical signal is a reproduction of the scanned areas of said charge replica capable of being converted into a visual presentation of said Xray image.

5. An X-ray image intensifier system for presenting a continuous visual presentation of a periodic X-ray image, said system comprising means for generating an electr-on beam of elemental cross sectional area; a target element including a thin glass pane, a tiuoroscopic s-creen disposed on one side thereof for converting the X-ray image into a light image, a transparent conductive coating disposed on the other side thereof, and a photoconductive layer with secondary electron emission characteristics having first and second surfaces with capacitance therebetween, said first surface being in contact with said conductive coating and said second surface being exposed to said electron beam; means for scanning said second surf-ace with said electron beam to liberate more secondary electrons than beam electrons incident thereon; means for collecting said secondary electrons at a rst quiescent potential level to charge said second surface substantially to said first potential level; means for directing high energy electrons uniformly over the area of said second surface concurrently with the existence of the X-ray image to charge each elemental area thereof in a negative direction at a predetermined rate; means for maintaining said conductive coating at a second quiescent potential level positive with respect to said first potential level to produce a potential gradient across said photoconduct-ive layer whereby the light intensity of each elemental area of said ligh-t image releases photo-electrons from corresponding elemental areas of said first surface in proportion to the light intensity thereon and said potential gradient removes the released photo-electrons away from said first surface thereby producing holes which migrate towards said secon-d surface t-o charge each elemental area thereof in a positive direction at a rate les-s than said predetermined rate and proportional to the ligh-t intensity on the corresponding elemental area of said first surface to produce a charge replica of the X-ray image, the secondary electrons collected at sra-id first potential level constituting a first electrical signal representative of the charge replica; means for amplifying said first electrical signal t-o produce a second electrical signal; means for impressing said second electrical signal on said conductive coating to shift the poten-tial of the elementalarea of said second surface Ibeing scanned by said electron beam substantially to said first quiescent poten-tial whereby said charge replica is not affected and said second electrical signal is a reproduction `of the scanned areas of said charge replica capable of being converted into a visual presentation of said X-ray image.

References Cited in the file of this patent UNITED STATES PATENTS 2,525,832 Sheldon Oc-t. 17, 1950 2,550,316 Wilder Apr, 24, 1951 2,654,852 Goodrich Oct. 6, 1953 

